Seasonal and diurnal variation of nitrogenase activity (acetylene reduction) in barrel medic (Medicago truncatula Gaertn.) grown in pots

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Abstract
Seasonal and diurnal variation of nitrogenase activity in Medicago truncatula Gaertn. was measured by means of the acetylene reduction assay on plants grown in pots. In a glasshouse set at 20°C, the seasonal pattern of acetylene reduction (AR) activity was closely correlated with dry weight and photosynthetic area. Short-term fluctuations in AR activity were mainly associated with irradiance. Measurements made of the diurnal variation of AR activity showed that rates of AR at noon were 10–60% (average 33%) higher than the mean daily rates. Effects of defoliation and shading gave further evidence for the importance of light and recent photosynthate for nitrogen fixation in root nodules of legumes. Values for acetylene reduction integrated over time were highly correlated with the total amount of nitrogen in the plant. The molar ratio of acetylene reduced to nitrogen accumulated by the plants at the end of the experiment was 1.2 : 1. The significance of this value is discussed. Multiple use of the same plant material to study the time course of nitrogen fixation by the AR assay was found to be feasible under certain conditions.

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